Oxidation Numbers — Rules Mnemonic
Target: mnemonic for oxidation number rules chemistry
Why is this hard to memorize?
Oxidation numbers (oxidation states) track electron gain/loss in compounds. The rules are: free elements = 0, monoatomic ions = charge, O = −2 (except peroxides −1, OF₂ +2), H = +1 (except metal hydrides −1), F = always −1, and sum of oxidation numbers = charge of species. These rules let you identify which atom is oxidized (increase in ON) and which is reduced (decrease in ON) in any reaction. NEET and JEE test oxidation number calculation, identifying redox reactions, and finding oxidizing/reducing agents.
Classic mnemonics you should know
"F first(−1 always) → O(−2) → H(+1) → others. Free element = 0. Sum = charge."
Assign in order of priority: F is always −1 (most electronegative). O is −2 (except: −1 in peroxides, +2 in OF₂). H is +1 (except: −1 in NaH, CaH₂). Then calculate the remaining atom algebraically. Sum of all oxidation numbers = net charge of the species.
"LEO the lion says GER: Lose Electrons = Oxidation, Gain Electrons = Reduction"
If oxidation number increases → atom lost electrons → OXIDIZED (reducing agent). If oxidation number decreases → atom gained electrons → REDUCED (oxidizing agent). The substance that gets oxidized is the reducing agent (it reduces others).
"O = −1 in peroxides (H₂O₂, Na₂O₂). O = +2 in OF₂. H = −1 in metal hydrides (NaH, CaH₂)."
These three exceptions are the most tested: In H₂O₂ → O is −1 (not −2). In OF₂ → O is +2 (F takes priority as −1). In NaH → H is −1 (Na is +1, must balance). Know these to avoid calculation errors.
The complete list
- Free element: ON = 0
- Monoatomic ion: ON = charge
- F = always −1
- O = −2 (except peroxides −1, OF₂ +2)
- H = +1 (except metal hydrides −1)
- Sum of ON = charge of species
- ON increases → oxidation (loses e⁻)
- ON decreases → reduction (gains e⁻)
Frequently asked questions
How do I find the oxidation number of an atom in a compound?
Assign known oxidation numbers first (F=−1, O=−2, H=+1, metals in their common state). Then use the rule that sum = charge to solve for the unknown. Example: In KMnO₄ → K=+1, O=−2, sum=0. So: +1 + Mn + 4(−2) = 0 → Mn = +7.
What is a disproportionation reaction?
A reaction where the SAME element is simultaneously oxidized AND reduced. Example: 2H₂O₂ → 2H₂O + O₂. Oxygen in H₂O₂ is −1. In H₂O it becomes −2 (reduced). In O₂ it becomes 0 (oxidized). The same element (O) goes in two directions. Also: Cl₂ + 2NaOH → NaCl + NaClO + H₂O (Cl goes from 0 to −1 and +1).
How do I identify the oxidizing and reducing agent?
The oxidizing agent GETS REDUCED (its own oxidation number decreases — it gains electrons from others). The reducing agent GETS OXIDIZED (its oxidation number increases — it gives electrons). Memory trick: the oxidizing agent oxidizes others but is itself reduced. Example: In 2Mg + O₂ → 2MgO, O₂ is the oxidizing agent (O: 0 → −2, reduced) and Mg is the reducing agent (Mg: 0 → +2, oxidized).
Explore more Chemistry mnemonics
We have 100+ memory tricks for Chemistry — periodic table groups, reactions, exceptions, and more.
Browse all Chemistry mnemonics